- #1
cpburris
Gold Member
- 38
- 4
Homework Statement
Show that the stability condition for a circular orbit of radius a, i.e.
[itex]f(a) + \frac{a}{3} (\frac{df}{dr})_{r=a} < 0 [/itex]
is equivalent to the condition
[itex] \frac{d^2V(r)}{dr^2} > 0 [/itex]
for r=a where V(r) is the effective potential given by
[itex] V(r) = U(r) + \frac{ml^2}{2r^2} [/itex]
The Attempt at a Solution
I understand fully why they are equivalent, and I would have no problem proving individually how each is a condition for stability, but analytically I really don't know how to show the two are equivalent. I'm not even sure what the question is asking. I tried just setting
[itex] -\frac{d^2V(r)}{dr^2} = f(a) + \frac{a}{3} (\frac{df}{dr})_{r=a} [/itex]
and do something from there, but it didn't get me anywhere.